Laminar structures of polyimides and fluorinated polymers, such as laminates of Kapton.RTM. polyimide film and fluorinated ethylenepropylene copolymers (FEP), are commercially available and are characterized by a unique combination of physical properties not separately possessed by each individual component. An important use of such laminar structures is as an insulation wrap for electrical conductors. However, a drawback of such laminar structures resides in the lack of sufficient bond strength and durability when exposed to high temperature environments. In addition, when such laminar structures are heat-sealed onto metal conductors, they cannot be readily removed or stripped from the conductor. In many end-uses, it is important that the insulation be readily strippable from the end of the metal conductor without damaging the conductor by nicking or scoring. The spirally wound insulation wrap, usually applied to the conductor in film or tape form and heat-sealed thereon, experiences some degree of shrinkage during the heat-sealing step and is quite difficult to remove. Both conventional polyimide/FEP and FEP/polyimide/FEP insulation tapes have poor strippability from the conductor since, in the case of polyimide against the metal, the coefficient of friction is high; and in the case of FEP against the metal, the FEP tends to bond to the metal surface during the heat-sealing step.
Moreover, while laminar structures containing polyimide have a very desirable balance of properties, such as high temperature and electrical properties, under some conditions, polyimide-containing structures can arc track, resulting in the undesirable formation of a carbonaceous char. Arc tracking is a catastrophic failure in the presence of an electrical arc when a short circuit occurs between the conductor and a conducting medium external to the insulation, such as a moderately conductive fluid. Such a failure causes mechanical damage to the insulation which rapidly propagates at the elevated temperature of the electrical arc.
U.S. Pat. No. 3,616,177, issued on Oct. 26, 1971, discloses an asymmetric, four-layer laminar structure, for use as an insulation wrapping for wire and cable, comprising a polyimide base layer, inner layers of FEP copolymer adhered to both sides of the polyimide base layer and an outer layer of a blend of PTFE and FEP adhered to one of the FEP inner layers. The laminar structures exhibit superior bond strength retention under conditions of high temperature and high humidity and are easily strippable from the electrical conductor. The single outer PTFE/FEP layer is located next to the conductor and provides the requisite strippability, whereas the opposite side of the structure utilizes a heat-sealable FEP layer to impart maximum sealability. However, the laminar structures do not contain a second PTFE/FEP layer nor outer FEP layers, and do not provide adequate arc track resistance.
U.S. Pat. No. 5,106,673, issued on Apr. 21, 1992, discloses a multi-layer laminar structure having improved adhesion and arc-tracking comprising a polyimide base layer, layers of PTFE/FEP blend adhered to both sides of the polyimide base layer and outer layers of PTFE adhered to both inner PTFE/FEP blend layers. The patent specifically teaches that blends of PTFE and FEP provide greater adhesion than FEP alone when used to bond the polyimide base layer and the outer PTFE layers. In contrast, the laminar structures of the present invention use a FEP layer and not a PTFE/FEP blend layer to bond the polyimide to a PTFE/FEP layer. An additional FEP coating also provides excellent film-to-film adhesion during the wrapping and heat-sealing process, as well as good strippability from the conductor.
Thus, a need still exists for a polyimide-fluoropolymer laminar structure which is both self-bonding and bondable to other insulation materials, has arc track propagation resistance and is strippable from the wire conductor while still retaining adequate adhesion to the conductor.